Exhaust Diameter Calculator

Use the engine's RPM and cylinder's volume to calculate the exhaust diameter of your vehicle.

Exhaust pipe inside diameter71.218 mm2.8039 inches for a single collector carrying the whole flow at 75 m/s

Intake volume flow90 litres/sdisplacement × rpm ÷ 2 × volumetric efficiency — the divide by two is the four-stroke cycle, which fills once per two revolutions

Exhaust volume flow298.7668 litres/s3.32× the intake flow purely from the temperature rise. That expansion is why exhaust pipes are larger than intake runners

Required cross-section39.8356 cm²

Primary pipe diameter, one per cylinder35.609 mm1.4019 inches for each of 4 primaries — but each fires only part of the time, so real primaries run larger than this steady-flow figure

At 10% higher rpm74.694 mmdiameter goes as the SQUARE ROOT of flow, so it changes slowly — which is why a handful of pipe sizes covers most engines

Bigger is not bettervelocity mattersan oversized pipe drops the gas velocity and loses the scavenging effect, where the pulse leaving one cylinder helps draw the next one clear. That is the physical reason a too-large exhaust costs low-end torque

This is steady-flow sizingreal exhausts are pulsedgas leaves in discrete slugs, and tuned-length systems exploit the pressure waves they create. Treat this as a starting diameter, not a finished design

The formula

Q = VNη/2; A = Q/v; D = √(4A/π)

Sized by velocity, not guesswork

A four-stroke engine ingests its displacement once every two revolutions, so the volume flow is displacement times rpm over two, corrected for volumetric efficiency. The exhaust is far hotter and has expanded in proportion to absolute temperature — typically threefold — which is why exhaust pipes are larger than intake runners on the same engine.

Choosing a target gas velocity around 75 m/s fixes the area, and the diameter follows. Because diameter goes as the square root of flow it changes slowly: a ten percent rpm increase needs under five percent more diameter.

Oversizing costs torque

A pipe that is too large drops the gas velocity and destroys the scavenging effect, where the departing pulse from one cylinder helps pull the next one clear. That is the physical reason a huge exhaust hurts low-end torque rather than helping, and why sizing is done on velocity rather than on flow capacity alone.

This is steady-flow sizing. Real exhaust leaves in discrete slugs, and tuned-length systems exploit the pressure waves they create, so treat the answer as a starting diameter rather than a finished design.